From e0eb7ca2dd07ca8878b67cc20e75f9d7e46d80f4 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Noralf=20Tr=C3=B8nnes?= <noralf@tronnes.org>
Date: Sat, 3 Jul 2021 16:13:21 +0200
Subject: [PATCH] drm/gud: Add async_flush module parameter
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[ drm-misc commit a0356899ebe8ecde0da9c5685cc47154db973a5e ]

Provide a way for userspace to choose synchronous flushing/pageflips.
This helps save CPU and power.

It is also useful for test scripts since userspace can know when a flush
has happended and wait before doing the next visual test.

Cc: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Reviewed-by: Peter Stuge <peter@stuge.se>
Link: https://patchwork.freedesktop.org/patch/msgid/20210703141321.35494-2-noralf@tronnes.org
---
 drivers/gpu/drm/gud/gud_pipe.c | 15 +++++++++++++++
 1 file changed, 15 insertions(+)

--- a/drivers/gpu/drm/gud/gud_pipe.c
+++ b/drivers/gpu/drm/gud/gud_pipe.c
@@ -24,6 +24,19 @@
 #include "gud_internal.h"
 
 /*
+ * Some userspace rendering loops runs all displays in the same loop.
+ * This means that a fast display will have to wait for a slow one.
+ * For this reason gud does flushing asynchronous by default.
+ * The down side is that in e.g. a single display setup userspace thinks
+ * the display is insanely fast since the driver reports back immediately
+ * that the flush/pageflip is done. This wastes CPU and power.
+ * Such users might want to set this module parameter to false.
+ */
+static bool gud_async_flush = true;
+module_param_named(async_flush, gud_async_flush, bool, 0644);
+MODULE_PARM_DESC(async_flush, "Enable asynchronous flushing [default=true]");
+
+/*
  * FIXME: The driver is probably broken on Big Endian machines.
  * See discussion:
  * https://lore.kernel.org/dri-devel/CAKb7UvihLX0hgBOP3VBG7O+atwZcUVCPVuBdfmDMpg0NjXe-cQ@mail.gmail.com/
@@ -577,6 +590,8 @@ void gud_pipe_update(struct drm_simple_d
 		if (gdrm->flags & GUD_DISPLAY_FLAG_FULL_UPDATE)
 			drm_rect_init(&damage, 0, 0, fb->width, fb->height);
 		gud_fb_queue_damage(gdrm, fb, &damage);
+		if (!gud_async_flush)
+			flush_work(&gdrm->work);
 	}
 
 	if (!crtc->state->enable)
